Transient absorption of inhaled vapors into a multilayer mucus-tissue-blood system

Previous studies have approximated the absorption of vapors into the walls of the respiratory tract as a steady state process. However, non-dimensional analysis indicates that the absorption of vapors in the conducting airways is time-dependent over the timescale of a breathing cycle. The objective of this study was to evaluate the mass transport of sample chemical species through a simple multilayer system composed of mucus, tissue, and blood components on a transient basis. Individual multilayer models were considered that represent the wall dimensions of the nasal extrathoracic (ET(2)), bronchial (BB), and bronchiolar (bb) airways. Sample vapors considered were acetaldehyde and benzene, which are highly soluble and moderately soluble in mucus, respectively. To determine absorption, mass transport was calculated based on an existing analytical steady state solution, a new analytical transient solution, and a numerical transient solution. Results indicated that concentrations within the mucus and tissue layers were highly time dependent in the ET(2) and BB regions and moderately time dependent in the bb airways over the timescale of an inhalation cycle, which is approximately 1-2 s. Fluxes of vapors into the tissue and blood varied with time for approximately 6-8 s in the BB region and 0.6-0.8 s in the bb model. The associated transient blood uptake of acetaldehyde and benzene in the upper ET(2) and BB regions varied from steady state values by a factor of approximately 30 after 1 s. Under similar conditions, transient uptake in the bb model varied from steady state conditions by a factor of approximately 1.3. Surprisingly, inclusion of chemical reactions in the mucus and tissue modified the transient uptake predictions only for very large values of reaction rate coefficients (K > 100 min(-1)). In summary, transient effects significantly impact the absorption of vapors into the walls of the upper respiratory tract (ET(2) and BB regions) and may largely diminish the effects of chemical reactions over the timescale of an inhalation cycle. Furthermore, the transient analytical solution that was developed provides the basis for an improved boundary condition in future CFD simulations of air-phase transport and wall absorption.